This invention relates in general to the fabrication of hybrid microelectronic circuits. More specifically, the invention relates to the fabrication of microstrip lines using a thick film process on a hybrid circuit substrate. It provides a novel fabrication technique that requires less labor and less specialized equipment than required for known thin film techniques and overcomes many of the difficulties associated with known thick film fabrication techniques.
Some of the problems attendant the fabrication of microwave hybrid circuits are discussed in a publication entitled THE MICROWAVE HYBRID CIRCUIT:FABRICATION-PROCESSING CONSIDERATIONS by William J. MacDonald of Film Microelectronics, Inc., 17 A Street Burlington, Mass. and Charles A. Wheeler of Sanders Associates, Inc. Microwave Division, Nashua, N.H. The authors point out that R.F. current flowing through a film conductor may experience the "skin effect" phenomenon. Current concentration at the substrate (ceramic) metallization interface places certain requirements on film (conductor) thickness and the bonding mechanism by which the film adheres.
At microwave frequencies the conduction characteristics of a film line are a function, at least in part of line width and edge definition as well as line thickness. Using known copper plated silver and other thick film copper techniques, it is possible to fabricate a microstrip resonator line having sufficiently low loss and high-Q as now required by modern circuit designs. However, there is insufficient ability to accurately control both the width, edge definition and thickness of the thick film line applied to the substrate. Using known techniques, when multiple layers of copper are printed to develop the desired skin depth thickness, the overlapping of layers destroys any clear definition of edge and the width of the microstrip line becomes uncertain. This reduces the predictability of microstrip line characteristics.
It is possible to achieve good width, edge definition and the thickness control using thin film techniques in which chromium-gold or chromium-nickel-gold is sputtered onto the substrate. However, the use of thin film techniques requires elaborate and expensive machinery. The costs are prohibitive except for mass production.
An exemplary known thin film process for producing thin film microstrip lines includes the following method steps:
1. Vacuum deposit, i.e., either evaporate or sputter, approximately 500 Angstroms of titanium onto a 99.5% Al.sub.2 O.sub.3 substrate. PA1 2. Vacuum deposit copper until the total thickness is 2 microns. This produces a phased Cu Ti metallized layer which has high adhesion to the alumina substrate. PA1 3. Copper is then electroplated onto the deposited metallization to increase the metal thickness to 27 microns, i.e, 25 microns of Cu is electrodeposited. PA1 4. The desired pattern is then photoproduced using a liquid photoresist, exposing and developing to leave openings in the photoresist where the pattern is to remain. PA1 5. 10-12 microns of gold is then electroplated onto the exposed copper. PA1 6. The photoresist is removed leaving a solid plane of copper with a gold pattern plated onto it. PA1 7. The copper and titanium are then etched from the substrate. The gold pattern acts as a mask so that the copper and titanium under the gold are not etched. PA1 500 Angstrom Titanium PA1 19,500 Angstrom Vacuum Deposited Copper PA1 25 Microns Electroplated Copper PA1 10-12 Microns Electroplated Gold PA1 U.S. Pat. No. 4,152,679--Chen (May 1, 1979) PA1 U.S. Pat. No. 3,808,049--Caley et al (Apr. 30, 1978) PA1 U.S. Pat. No. 3,274,328--Davis (Sept. 20, 1966) PA1 U.S. Pat. No. 2,257,629--Kornreich (June 21, 1966)
The resulting metallized pattern therefor includes
The process steps required to produce a microstrip line of this nature requires a significant amount of labor and specialized equipment. The thick film technique set forth herein is more simple and requires less labor and specialized equipment.
A small sample of U.S. patents illustrate known techniques for fabricating microelectronic circuits as follows:
The subject matter of these patents are incorporated herein by reference. This is not intended to be an exhaustive list but only a small sample of the U.S. patents issued in the general art area to which this invention pertains.